1. Field of the Invention
This invention relates to density calibration tools for setting the density output of printers, and more specifically, for using a print-out from a standard printer with known and controlled settings as a standard for comparison with output printed by a target printer sought to be calibrated.
2. Description of the Related Art
Images of varying "shades" can be printed using only a black toner or ink. The "shades" are produced by printing varying gray levels of black from 0 to 100% coverage. "Shades" or "gray levels" are also referred to as half-tones. A half-tone algorithm can be used by a printer for instructing that printer as to which "pel" or dot of ink to print for varying coverage amounts. For example, for 100% coverage, or darkest image, the printer would print a dot in every single pel. For 50% coverage, a dot would be printed in every other pel. Certain patterns of dots are used for the various amounts of coverage. Lighter half-tones or gray levels are produced as the coverage amounts decrease.
However, another factor that affects the lightness or darkness of an image is optical density. In other words, even though a part of an image may have 100% black coverage, that part of the image may appear lighter or darker depending upon its optical density.
For example, with 100% coverage using a toner or ink that is 100% perfectly black, there would be a reflectance value of zero (0), i.e., no light would be reflected back. The formula for optical density is the log of the background divided by the reflectance of the spot that is being examined. In other words, if the reflectance had a value of 10 (which is pretty dark), but the background is white having 90% reflectance, i.e., having a value of 90, the ratio is 9. The log to the base 10 of 9 is slightly less than 1, i.e., 0.954, which would be the value of the optical density.
In order to get good images when printing, the optical density of the printed toners has to be maintained. If optical density changes in the printer, then the images will change and the image quality may not be as good as desired. In high function and high cost printers, it is known to have a densitometer built into the printer to either completely automate the control of the density, or to semi-automate density control by including operator interactions. However, for low cost printers, the cost of having a densitometer, either built into the printer or available to the operator, is prohibitive. For example, a densitometer may cost one thousand dollars, but many low cost printers sell for less than this amount. A printer manufacturer can not afford to build a densitometer into a low cost printer and still keep the printer "low-cost". Likewise, most users of "low-cost" printers cannot afford the price of a densitometer as an extra tool for maintaining the optical density of the printer. Nevertheless, even for low cost printers there is a need to maintain optical density in order to provide quality print images.
In essence, monochrome halftone rendering is inconsistent among printers due to differences in the optical density produced by the printer for various gray scale levels. A densitometer is typically used to calibrate the printer to the correct optical density. However, this tool is not available to the average customer. There is a need to maintain image quality by maintaining optical density without using a densitometer.